Next Leg of NPR Labs Elevated Power Testing Is Complete

Minnesota tests related to the proposed voluntary FM IBOC power increase are complete and NPR Labs is now analyzing that data.

“Everybody wants an answer about how to manage power in light of adjacent neighbors,” John Kean of NPR Labs tells me.

Harris shipped an HPX40 high-power, common-amplification transmitter up to KCRB(FM), Bemidji, Minn. for the elevated power tests, which took place on three days and two nights in mid-June. Engineers raised the digital power for the Minnesota Public Radio station at its transmitter site in Black Duck to –20 dB, –14 dB and –10 dB while NPR Labs took mobile field measurements from both KCRB and on so-called “victim station” KBPN(FM), Brainerd, Minn. with test gear in its measurement vehicle.

KBPN is a first-adjacent to KCRB and 75 miles to the southeast. The idea of this “pilot” test was to see how raising the IBOC transmission power on KCRB affected mobile reception of KBPN as samples of loud and soft music and male and female talk were aired.

Engineers took audio recordings and field strength measurements for each test. Kean and Jan Andrews of NPR Labs conducted the testing along with MPR engineers Mike Hendrickson and Doug Thompson. CBS Radio DOE Glynn Walden, who’s a member of the working group that’s advising NPR on the tests, rode in the car and coordinated transmission powers and cued audio samples from KCRB and KBPN. Harris’ Tim Anderson, Brett Blankenship and Terry Cockerill helped with the installation and transmitter testing.

The results are now being tabulated, Kean said, and in the next week the audio samples will be evaluated by listeners in a controlled subjective test. Kean says the material gathered in Minnesota and in Rhode Island in May will determine if the field audio and lab-generated samples are “statistically indistinguishable,” as he says. It’s an important validation, John says, to use lab-generated samples for the in-car testing slated for a Baltimore suburb yet this month, which will determine the IBOC interference ratios for the project.